Can the opening size of a fireplace be reduced? Could the facing of the
granite fireplace surround be made to cover part of the length of the
opening: i.e. 29"hx48" wide, reduce this to 36" wide? We could use fire
bricks to make the opening smaller.
How do we measure the size of the flue? We cannot tell by looking up past
the damper as the walls are slanted. Do we have to get up on the roof?
Thank you
Fireplace Design & Dimensions
Summary: To prevent problems resulting from chimney construction and fireplace design, check out the following resources. Because firebox design is critical to the success of the fireplace, consult the fireplace sizing chart before building.
Fireplace Design Literature
The Brick Industry Association (http://www.bia.org) has two excellent pamphlets about fireplace and chimney design and construction. Go to their web site and find the section on Technical Notes. Scroll down the left hand list of articles and click on the following titles:
#19 Residential Fireplace Design
#19A Residential Fireplaces, Details and Construction
Fireplace Sizing Table
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All of the following dimensions are given in inches, except for Chimney Height. That value is given in feet. The firebox design is critical. Here is a photo of a new masonry firebox in my own home. The rear wall of the firebox rises vertically from the floor of the fireplace and then begins to slant towards the fireplace opening just at the top of the gas logs. The sidewalls of the firebox are vertical. Note that the firebox is not the same width front to back. It gets narrower towards the rear of the firebox!
The firebox depth does not include the thickness of the brick - or marble in my case - that faces your fireplace. This is very important! You can mix and match flue sizes with chimney heights. To properly size a flue, you must use a Nomograph. See the Brick Industry Association's Technical Note 19B - Residential Chimneys. It is always better to use the smaller flue size when you have two choices. When using the smaller flue size, you will be required to raise the height of the chimney. Taller chimneys draw better than short chimneys of the same flue size.
The smoke chamber height and design is very critical. You generally can't see this space as it is hidden above your fireplace damper. The height of the damper is very critical as well. The bottom of the damper should be a minimum of 8 inches above the top of the fireplace opening. If your damper is not at least 8 inches above this spot, you will have problems for sure! I urge you to read the pamphlets I have mentioned from the Brick Industry Association. They have excellent illustrations which will enable you to clearly see what I am trying to explain! The following table and its values are courtesy of the Brick Industry Association.
| Fireplace Opening Width | 24" | 28" | 32" | 36" | 40" | 42" | 48" |
| Fireplace Opening Height | 24" | 24" | 29" | 29" | 29" | 32" | 32" |
| Firebox Depth | 16" | 16" | 16" | 16" | 16" | 16" | 18" |
| Rear Firebox Width | 11" | 15" | 19" | 23" | 27" | 29" | 33" |
| Rear Firebox Wall Vertical Height | 14" | 14" | 14" | 14" | 14" | 16" | 16" |
| Nominal Flue Size | 8 x 12 | 8 x 12 | 12 x 12 | 12 x 12 | 12 x 16 | 12 x 16 | 16 x 16 |
| Minimum Chimney Height | 15' | 20' | 18.5' | 20.5' | 13' | 19.5' | 16.5' |
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Comments:
Susie 28 Nov 2007, 17:46
ATB 28 Nov 2007, 17:52
Sue,
If you alter one measurement, all must be modified. You need to get on the roof to verify the flue size. Be careful!
Melvyn DAVIES 04 Dec 2007, 15:49
SIZE OF FLUE FOR:
Fireplace Opening Width 36" Fireplace Opening Height 30" Firebox Depth 16" Rear Firebox Width 24" Rear Firebox Wall Vertical Height 18-20" Minimum Chimney Height 50'
ted 06 Dec 2007, 21:21
Hello Ive built about hundred fireplaces
, but i never built one outside ,I'm WORRIED about the draft the contractor
has no prints but he wants it to be 36" box but only 8' tall .I don't want
to built it wrong . can you help me ? thanks ted
AsktheBuilder 07 Dec 2007, 05:15
Ted,
Sure. Read *all* of my articles about Fireplace Design. There are two or three critical articles. Pay very close attention to the table on this very page with all of the dimensions.
Keith 07 Dec 2007, 06:32
Tim,
I have a see through fireplace which I have come to realize since I bought the house is pretty but not practical. I've considered closing off one side and or raising the chimney height but would like to hear your opinion. This thing is a mess compared to the dimensions chart. Thanks, Keith opening height 26" both sides 38" wide both sides 31" deep 47" from bottom of firebox to damper 12x12 flue 14.5' chimney height from the damper
AsktheBuilder 07 Dec 2007, 06:39
Keith,
If you want a fireplace design that performs flawlessly, then you design a fireplace exactly to the dimensions in the table on this page. If this means getting out a sledge hammer to help you, then so be it. Sorry, but it was time for some tough love.
Steven 08 Dec 2007, 10:18
I am new to the north east. My house is about 20 years old. When I burn my
firepalce smokes goes down into my basement. There is a metal opening on
the freplaces floor that leads down to an door in the basement wall. Not
sure why this is happening? Any suggestions?
AsktheBuilder 08 Dec 2007, 11:07
Steven,
Please read all about backdrafting and makeup air here on AsktheBuilder.com.
Jim L. 10 Dec 2007, 18:34
Do you think a 46" W x 49" H x 29" D fireplace with a 16"x16" flue and a
chimney >16' tall will draft properly?
If not, how should I adjust the fireplace dimensions? Thanks View all comments |
